Any alloyed lead will cast larger than pure lead. I would not try to use WW metal in a .50, although in a slow twist .54, it'd probably work. You have to use a smaller mould. WW usually casts .003" to .004" larger than pure lead. Ohioan, perhaps your .490" mould is a bit larger than the indicated .490". For example, my .684" mould casts .680" in pure lead and .684" in WW metal.
The reason WW casts larger balls, is alloyed lead shrinks less than does pure lead. The harder the alloy, generally, the larger it casts. In the States, be careful of WW metal. When melting WW, most of the glue-on weights are almost pure lead, while some of the clamp-ons (& maybe some of the glue-ons) are made of zinc. Including zinc into your melt will destroy the entire batch. It casts poorly, and sometimes will even contaminate the lead pot as well.